Sonic hedgehog (SHH) is a member of the hedgehog family of signalling molecules. SHH was initially described as a protein secreted from two signalling centres, the notochord and the floor plate. Subsequently, it was identified as a morphogen that is directly responsible for dorso-ventral patterning of the CNS. More recently, additional sites of SHH expression have been identified and multiple actions of SHH during CNS development discovered, including the specification of oligodendrocytes, proliferation of neural precursors and control of axon growth. Despite these various activities, it appears that the SHH signalling pathway is well conserved and that the same mechanisms are utilized to achieve a variety of cellular responses. Therefore, a more precise understanding of the molecular mechanisms that underlie the different responses to SHH signalling is the next step in the study of this molecule and its role in the regulation of neural development.